From: Andy Lutomirski <luto@amacapital.net>
To: Khalid Aziz <khalid.aziz@oracle.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
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Subject: Re: [RFC PATCH v9 12/13] xpfo, mm: Defer TLB flushes for non-current CPUs (x86 only)
Date: Fri, 5 Apr 2019 09:24:01 -0600 [thread overview]
Message-ID: <C1253C86-DD1F-469F-9B5E-ED7AA9FBEE4D@amacapital.net> (raw)
Message-ID: <20190405152401.09Tg5SOU4YLQ4aOp95oWHKv41_R8jw8mhACDQWlqOro@z> (raw)
In-Reply-To: <91f1dbce-332e-25d1-15f6-0e9cfc8b797b@oracle.com>
>>> On Apr 4, 2019, at 4:55 PM, Khalid Aziz <khalid.aziz@oracle.com> wrote:
>>>
>>> On 4/3/19 10:10 PM, Andy Lutomirski wrote:
>>> On Wed, Apr 3, 2019 at 10:36 AM Khalid Aziz <khalid.aziz@oracle.com> wrote:
>>>
>>> XPFO flushes kernel space TLB entries for pages that are now mapped
>>> in userspace on not only the current CPU but also all other CPUs
>>> synchronously. Processes on each core allocating pages causes a
>>> flood of IPI messages to all other cores to flush TLB entries.
>>> Many of these messages are to flush the entire TLB on the core if
>>> the number of entries being flushed from local core exceeds
>>> tlb_single_page_flush_ceiling. The cost of TLB flush caused by
>>> unmapping pages from physmap goes up dramatically on machines with
>>> high core count.
>>>
>>> This patch flushes relevant TLB entries for current process or
>>> entire TLB depending upon number of entries for the current CPU
>>> and posts a pending TLB flush on all other CPUs when a page is
>>> unmapped from kernel space and mapped in userspace. Each core
>>> checks the pending TLB flush flag for itself on every context
>>> switch, flushes its TLB if the flag is set and clears it.
>>> This patch potentially aggregates multiple TLB flushes into one.
>>> This has very significant impact especially on machines with large
>>> core counts.
>>
>> Why is this a reasonable strategy?
>
> Ideally when pages are unmapped from physmap, all CPUs would be sent IPI
> synchronously to flush TLB entry for those pages immediately. This may
> be ideal from correctness and consistency point of view, but it also
> results in IPI storm and repeated TLB flushes on all processors. Any
> time a page is allocated to userspace, we are going to go through this
> and it is very expensive. On a 96-core server, performance degradation
> is 26x!!
Indeed. XPFO is expensive.
>
> When xpfo unmaps a page from physmap only (after mapping the page in
> userspace in response to an allocation request from userspace) on one
> processor, there is a small window of opportunity for ret2dir attack on
> other cpus until the TLB entry in physmap for the unmapped pages on
> other cpus is cleared.
Why do you think this window is small? Intervals of seconds to months between context switches aren’t unheard of.
And why is a small window like this even helpful? For a ret2dir attack, you just need to get CPU A to allocate a page and write the ret2dir payload and then get CPU B to return to it before context switching. This should be doable quite reliably.
So I don’t really have a suggestion, but I think that a 44% regression to get a weak defense like this doesn’t seem worthwhile. I bet that any of a number of CFI techniques (RAP-like or otherwise) will be cheaper and protect against ret2dir better. And they’ll also protect against using other kernel memory as a stack buffer. There are plenty of those — think pipe buffers, network buffers, any page cache not covered by XPFO, XMM/YMM saved state, etc.
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next prev parent reply other threads:[~2019-04-05 15:24 UTC|newest]
Thread overview: 102+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-03 17:34 [RFC PATCH v9 00/13] Add support for eXclusive Page Frame Ownership Khalid Aziz
2019-04-03 17:34 ` [RFC PATCH v9 11/13] xpfo, mm: optimize spinlock usage in xpfo_kunmap Khalid Aziz
[not found] ` <5bab13e12d4215112ad2180106cc6bb9b513754a.1554248002.git.khalid.aziz-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>
2019-04-04 7:56 ` Peter Zijlstra
2019-04-04 16:06 ` Khalid Aziz
[not found] ` <cover.1554248001.git.khalid.aziz-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>
2019-04-03 17:34 ` [RFC PATCH v9 01/13] mm: add MAP_HUGETLB support to vm_mmap Khalid Aziz
2019-04-03 17:34 ` [RFC PATCH v9 02/13] x86: always set IF before oopsing from page fault Khalid Aziz
[not found] ` <e6c57f675e5b53d4de266412aa526b7660c47918.1554248002.git.khalid.aziz-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>
2019-04-04 0:12 ` Andy Lutomirski
[not found] ` <CALCETrXvwuwkVSJ+S5s7wTBkNNj3fRVxpx9BvsXWrT=3ZdRnCw-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2019-04-04 1:42 ` Tycho Andersen
2019-04-04 4:12 ` Andy Lutomirski
[not found] ` <CALCETrVp37Xo3EMHkeedP1zxUMf9og=mceBa8c55e1F4G1DRSQ-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2019-04-04 15:47 ` Tycho Andersen
2019-04-04 16:23 ` Sebastian Andrzej Siewior
2019-04-04 16:28 ` Thomas Gleixner
[not found] ` <alpine.DEB.2.21.1904041822320.1802-ecDvlHI5BZPZikZi3RtOZ1XZhhPuCNm+@public.gmane.org>
2019-04-04 17:11 ` Andy Lutomirski
2019-04-03 17:34 ` [RFC PATCH v9 03/13] mm: Add support for eXclusive Page Frame Ownership (XPFO) Khalid Aziz
[not found] ` <f1ac3700970365fb979533294774af0b0dd84b3b.1554248002.git.khalid.aziz-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>
2019-04-04 7:21 ` Peter Zijlstra
[not found] ` <20190404072152.GN4038-Nxj+rRp3nVydTX5a5knrm8zTDFooKrT+cvkQGrU6aU0@public.gmane.org>
2019-04-04 9:25 ` Peter Zijlstra
2019-04-04 14:48 ` Tycho Andersen
2019-04-04 7:43 ` Peter Zijlstra
[not found] ` <20190404074323.GO4038-Nxj+rRp3nVydTX5a5knrm8zTDFooKrT+cvkQGrU6aU0@public.gmane.org>
2019-04-04 15:15 ` Khalid Aziz
[not found] ` <b414bacc-2883-1914-38ec-3d8f4a032e10-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>
2019-04-04 17:01 ` Peter Zijlstra
2019-04-17 16:15 ` Ingo Molnar
2019-04-17 16:15 ` Ingo Molnar
[not found] ` <20190417161042.GA43453-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2019-04-17 16:49 ` Khalid Aziz
2019-04-17 16:49 ` Khalid Aziz
2019-04-17 17:09 ` Ingo Molnar
2019-04-17 17:09 ` Ingo Molnar
2019-04-17 17:19 ` Nadav Amit
2019-04-17 17:19 ` Nadav Amit
2019-04-17 17:26 ` Ingo Molnar
2019-04-17 17:26 ` Ingo Molnar
2019-04-17 17:44 ` Nadav Amit
2019-04-17 17:44 ` Nadav Amit
2019-04-17 21:19 ` Thomas Gleixner
2019-04-17 21:19 ` Thomas Gleixner
[not found] ` <alpine.DEB.2.21.1904172317460.3174-ecDvlHI5BZPZikZi3RtOZ1XZhhPuCNm+@public.gmane.org>
2019-04-17 23:18 ` Linus Torvalds
2019-04-17 23:18 ` Linus Torvalds
2019-04-17 23:42 ` Thomas Gleixner
2019-04-17 23:42 ` Thomas Gleixner
2019-04-17 23:52 ` Linus Torvalds
2019-04-17 23:52 ` Linus Torvalds
2019-04-18 4:41 ` Andy Lutomirski
2019-04-18 4:41 ` Andy Lutomirski
2019-04-18 5:41 ` Kees Cook
2019-04-18 5:41 ` Kees Cook via iommu
2019-04-18 14:34 ` Khalid Aziz
2019-04-18 14:34 ` Khalid Aziz
2019-04-22 19:30 ` Khalid Aziz
2019-04-22 19:30 ` Khalid Aziz
[not found] ` <8f9d059d-e720-cd24-faa6-45493fc012e0-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>
2019-04-22 22:23 ` Kees Cook via iommu
2019-04-22 22:23 ` Kees Cook via iommu
[not found] ` <CAHk-=whUwOjFW6RjHVM8kNOv1QVLJuHj2Dda0=mpLPdJ1UyatQ-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2019-04-18 6:14 ` Thomas Gleixner
2019-04-18 6:14 ` Thomas Gleixner
2019-04-17 17:33 ` Khalid Aziz
2019-04-17 17:33 ` Khalid Aziz
2019-04-17 19:49 ` Andy Lutomirski
2019-04-17 19:49 ` Andy Lutomirski
[not found] ` <CALCETrXFzWFMrV-zDa4QFjB=4WnC9RZmorBko65dLGhymDpeQw-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2019-04-17 19:52 ` Tycho Andersen
2019-04-17 19:52 ` Tycho Andersen
2019-04-17 20:12 ` Khalid Aziz
2019-04-17 20:12 ` Khalid Aziz
2019-05-01 14:49 ` Waiman Long
2019-05-01 14:49 ` Waiman Long
2019-05-01 15:18 ` Khalid Aziz
2019-05-01 15:18 ` Khalid Aziz
2019-04-03 17:34 ` [RFC PATCH v9 04/13] xpfo, x86: Add support for XPFO for x86-64 Khalid Aziz
[not found] ` <c15e7d09dfe3dfdb9947d39ed0ddd6573ff86dbf.1554248002.git.khalid.aziz-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>
2019-04-04 7:52 ` Peter Zijlstra
[not found] ` <20190404075206.GP4038-Nxj+rRp3nVydTX5a5knrm8zTDFooKrT+cvkQGrU6aU0@public.gmane.org>
2019-04-04 15:40 ` Khalid Aziz
2019-04-03 17:34 ` [RFC PATCH v9 05/13] mm: add a user_virt_to_phys symbol Khalid Aziz
2019-04-03 17:34 ` [RFC PATCH v9 06/13] lkdtm: Add test for XPFO Khalid Aziz
2019-04-03 17:34 ` [RFC PATCH v9 07/13] arm64/mm: Add support " Khalid Aziz
2019-04-03 17:34 ` [RFC PATCH v9 08/13] swiotlb: Map the buffer if it was unmapped by XPFO Khalid Aziz
2019-04-03 17:34 ` [RFC PATCH v9 09/13] xpfo: add primitives for mapping underlying memory Khalid Aziz
2019-04-03 17:34 ` [RFC PATCH v9 10/13] arm64/mm, xpfo: temporarily map dcache regions Khalid Aziz
2019-04-03 17:34 ` [RFC PATCH v9 12/13] xpfo, mm: Defer TLB flushes for non-current CPUs (x86 only) Khalid Aziz
[not found] ` <4495dda4bfc4a06b3312cc4063915b306ecfaecb.1554248002.git.khalid.aziz-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>
2019-04-04 4:10 ` Andy Lutomirski
[not found] ` <CALCETrXMXxnWqN94d83UvGWhkD1BNWiwvH2vsUth1w0T3=0ywQ-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2019-04-04 22:55 ` Khalid Aziz
[not found] ` <91f1dbce-332e-25d1-15f6-0e9cfc8b797b-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>
2019-04-05 7:17 ` Thomas Gleixner
[not found] ` <alpine.DEB.2.21.1904050909520.1802-ecDvlHI5BZPZikZi3RtOZ1XZhhPuCNm+@public.gmane.org>
2019-04-05 14:44 ` Dave Hansen
[not found] ` <26b00051-b03c-9fce-1446-52f0d6ed52f8-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
2019-04-05 15:24 ` Andy Lutomirski
2019-04-05 15:24 ` Andy Lutomirski
[not found] ` <DFA69954-3F0F-4B79-A9B5-893D33D87E51-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org>
2019-04-05 15:56 ` Tycho Andersen
2019-04-05 15:56 ` Tycho Andersen
2019-04-05 16:32 ` Andy Lutomirski
2019-04-05 16:32 ` Andy Lutomirski
2019-04-05 15:56 ` Khalid Aziz
2019-04-05 15:56 ` Khalid Aziz
2019-04-05 16:01 ` Dave Hansen
2019-04-05 16:01 ` Dave Hansen
[not found] ` <36b999d4-adf6-08a3-2897-d77b9cba20f8-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
2019-04-05 16:27 ` Andy Lutomirski
2019-04-05 16:27 ` Andy Lutomirski
[not found] ` <E0BBD625-6FE0-4A8A-884B-E10FAFC3319E-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org>
2019-04-05 16:41 ` Peter Zijlstra
2019-04-05 16:41 ` Peter Zijlstra
2019-04-05 17:35 ` Khalid Aziz
2019-04-05 17:35 ` Khalid Aziz
2019-04-05 15:44 ` Khalid Aziz
2019-04-05 15:44 ` Khalid Aziz
2019-04-05 15:24 ` Andy Lutomirski [this message]
2019-04-05 15:24 ` Andy Lutomirski
2019-04-04 8:18 ` Peter Zijlstra
2019-04-03 17:34 ` [RFC PATCH v9 13/13] xpfo, mm: Optimize XPFO TLB flushes by batching them together Khalid Aziz
2019-04-06 6:40 ` [RFC PATCH v9 00/13] Add support for eXclusive Page Frame Ownership Jon Masters
2019-04-06 6:40 ` Jon Masters
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